1. Field of the Invention
The present invention relates to a preparation method of chiral nematic liquid crystal composition containing nematic liquid crystal material and (stacked) liquid crystal display.
2. Description of the Related Art
A liquid crystal device is basically constituted by a pair of substrates with transparent electrodes and a liquid crystal layer between the substrates. Driving voltage is applied to the liquid crystal layer to control alignment of liquid crystal molecules, so that incident light into device is modulated to display aimed images.
Various systems for liquid crystal display have been proposed. For example, liquid crystal display devices using chiral nematic liquid crystal composition have been studied, in which the chiral nematic liquid crystal composition is prepared to show cholesteric phase at room temperature and selective reflection in visible region. Such liquid crystal display device utilizes the fact that chiral nematic liquid crystal makes selective reflection of light having a specific wavelength and is well known as a liquid crystal device of reflection-type characterized by low power consumption. The display is made by switching liquid crystal conditions between planar state (colored state) and focal conic state (transparent state) by applying high and low pulse voltage. Even after the application of pulse voltage is stopped, the planar state and the focal conic state are kept (this property is called “memorizability”).
However, when just adjustment of chiral nematic liquid crystal composition is made so that cholesteric phase at room temperature and selective reflection in visible region can be shown, the following problems arise from the viewpoint of practical use.
The trial has been made to put a stacked liquid crystal device using additive color mixing in practical use so that a full color liquid crystal device can be realized. The stacked liquid crystal display device is generally constituted by a liquid crystal display device for displaying red color, a liquid crystal display device for displaying green color, and a liquid crystal display device for displaying blue color, each display for displaying each color being set up so that selective reflection of light of specified wavelength might be carried out.
Because the wavelength of selective reflection set up in each liquid crystal device shifts depending on change of circumference environment, especially ambient temperature, such a problem arises that light of specified wavelength is not reflected selectively and that desired color can not be realized.
From the viewpoint of decrease of production cost of stacked liquid crystal device, it is advantageous that driving power supply of each liquid crystal device is made common. Therefore, it is necessary to make driving properties (responsibility), especially threshold voltage, of each liquid crystal device approximate. In order to make driving properties approximate, it is necessary that anisotropy of dielectric constant of liquid crystal composition in each liquid crystal device is made approximate. It is known that anisotropy of dielectric constant of liquid crystal composition depending on an amount of addition of chiral material. However, when it is tried to adjust anisotropy of dielectric constant to a specified value only by amount of addition of chiral materials, there arises such a problem as selective reflection wavelength adjusted in liquid crystal composition shifts greatly from a predetermined value. Thus, in the conventional technology, a dielectric constant anisotropy was not able to be adjusted effectively, securing predetermined selective reflection wavelength.
When liquid crystal devices are continuously produced, there exists in general variability in manufacturing conditions caused by variability of composition in lot of materials constituting chiral nematic liquid crystal composition, such as nematic liquid crystal materials and chiral materials. Such variability of composition in lot of materials constituting chiral nematic liquid crystal composition causes variability of anisotropy of dielectric constant in the obtained liquid crystal compositions. The variability of anisotropy of dielectric constant and the variability of manufacturing conditions cause particularly variability of driving voltage in each liquid crystal devices constituting a stacked liquid crystal device. When each liquid crystal device constituting the stacked liquid crystal device is produced, it becomes necessary to adjust anisotropy of dielectric constant of liquid crystal composition in accordance with the above variability whenever conditions of cells or lots of liquid crystal composition materials change, so that driving properties of each liquid crystal device may be made approximate. However, as anisotropy of dielectric constant can not be effectively adjusted while securing a predetermined selective reflection wavelength as described above, it has been difficult to make driving power supply common in the stacked liquid crystal device.